A semiconductor device capable of holding data that includes a transistor whose semiconductor layer is formed using silicon (Si) and a transistor whose semiconductor layer is formed using an oxide semiconductor (Oxide Semiconductor: OS) has attracted attention (see Patent Document 1).
In recent years, with an increase in the amount of data manipulated, a semiconductor device having a large storage capacity has been required. In such a situation, the semiconductor device disclosed in Patent Document 1 has a structure in which electric charge is held using a transistor whose semiconductor layer includes an oxide semiconductor (OS transistor) to read the data.